1. Field of the Invention
The present invention relates to a method for laser cutting a manufactured object by means of a focused high energy laser beam, a system for precision processing and machining of materials, and manufactured objects having been cut by this method.
2. Description of the Related Art
The method of cutting manufactured objects by means of a focused laser beam is in general known in the art. Most commercial systems use a fiber optic bundle to deliver the beam to the cutting station. However, the use of fiber optics is totally unsuitable for precision cutting since the fiber optics very badly degrade the beam quality. A beam quality factor M2 (as defined below) for a fiber delivered beam is typically in the 50 to 150 range whereas a beam quality factor M2 of less than 2 is needed for precision cutting. Since M2 enters the equation for brightness (as defined below) as its square in the denominator, fiber coupled lasers fail the necessary brightness by orders of magnitude.
Only a few references discuss using a laser beam directly for cutting. Typical is Johnson et al., U.S. Pat. No. 5,057,664, which describes the use of a diode pumped, Q-switched YAG laser for cutting very thin (about 1 mm or less) films of materials having a high melting point, such as nichrome or tantalum nitride, as used for thin-film resistors. They report a trim profile with a good smoothness.
Basu et al., U.S. Pat. No. 4,890,289, describe a high energy diode pumped solid state laser arrangement applicable, e.g., for laser cutting. To avoid distortion of the laser gain material by heat generated together with the radiation, they propose to arrange the pumping diodes in a certain distance from the laser material, to guide and concentrate the pumping radiation by means of fiber optics, and to cyclically move the pumped region to distribute the heat generated in the laser by the optical pumping radiation. Such an arrangement needs obviously rather sophisticated mechanical and control means. Further, a slab laser, such as disclosed in Basu et al. is less preferred than a rod laser for precision cutting since a slab laser typically exhibits astigmatism which also prevents the beam quality factor M2 from being in the desired range.
Baer, U.S. Pat. 5,059,764, employs an end-pumped, fiber coupled, solid state rod laser pumped by laser diodes, for application to semiconductor manufacture and repair. The beam width in this device can be as low as 1-2 mm, and the pulses have typically an energy of 30 mJ. Single pulses are employed to ablate undesired links in semiconductor circuits.
The methods of Johnson et al. and Baer described above are useful for shaping of rather thin objects like metal layers in semiconductor circuits and the material is removed mainly by ablation. The method of Basu et al. is generally suitable for thicker materials but can only make relatively coarse cuts, i.e., about ten times wider than desirable. There remains a need for a method to make precision microcuts in thicker manufactured objects, i. e., of cuts having a high aspect ratio L/D between material thickness L and cut width D. Kobsa et al., U.S. Pat. No. 5,168,143, have employed a laser beam to cut complex capillaries in spinneret plates. Such plates are typically of metal and are 0.2 to 1.0 mm thick, although they can be as thin as 0.1 mm or as thick as 2.0 mm. In this process the focused laser energy creates a pool of molten material between the face (upper) and the bottom (lower) surface of the plate and the molten metal is expelled by means of a pressurized fluid flowing coaxially with the laser beam. The laser beam is substantially a single-mode beam and is focused to a spot size of less than 100 mm on a location above the upper surface of the plate. The laser is preferably pumped with xenon flash tubes and operated at a repetition frequency between 100 and 200 Hz. By this method, cuts about 40 mm wide and with satisfactory smooth edges could be made in metal and ceramic plates of some 0.2 to 1.3 mm thickness.
Specific to this method is the occurrence of a heat affected zone (HAZ) at the surface of the kerf. This may be caused by molten material which was not removed by the fluid jet but solidified due to heat dissipation in the manufactured object by recrystallization, or by the occurrence of cracks due to thermal stress or by other effects. The thickness of the HAZ created by known methods of laser cutting depends on material being cut and by process parameters and is typically 25 to 50 mm. Generally the material in the HAZ has different physical and chemical properties than the bulk material. Thus it may happen that when the manufactured object is in use the heat affected zone is less resistant to mechanical wear and/or to chemical attack than the bulk material and the dimension of the cut region may change rapidly when in use. This is obviously a disadvantage.
Therefore it is the object of the present invention to improve the laser cutting process in such manner that the thickness of the heat affected zone in a laser cut object is significantly less than that achieved by the prior art.
According to the invention there is provided a method for cutting through a manufactured object with a thickness of about 0.1 to 2.0 mm that includes the steps of using a laser having side-pumped diode bars, which has an average power output from about 2 to about 100 watts and delivers an average laser beam brightness of greater than 1012 W/m2*sr and a peak brightness of more than 1013 W/m2*sr to produce a laser beam. The laser beam is focused to a plane between the upper and the lower surface of the object, which either melts or vaporizes the material. The melted or vaporized material is expelled by the laser beam from the object by a pressurized fluid flowing coaxially with the laser beam.
The invention also comprises a laser based system for precision processing and machining of materials and workpieces. This system comprises a solid state laser light source comprising a laser medium, preferably a cylindrical rod of neodymium-doped yttrium aluminum garnet (Nd:YAG), a laser cavity defined by two end mirrors, a multiplicity of diode bars arranged around the laser medium for pumping it, and optical elements for improving the beam quality. Such optical elements are generally known and may be apertures for selecting single modes (e. g. the TEMoo mode), lenses, or curved mirrors. The system further comprises a lens system for expanding, collimating, and focusing the laser beam. The focusing lens may be located close to the collimator or at some distance to leave space for other parts of the system, like beam attenuators or beam splitters for beam diagnostic. The system further comprises a workstation for holding and moving a workpiece with respect to the laser beam. This workstation holds the workpiece, e. g., by clamping elements, and has means to independently move the workpiece in two orthogonal directions and to independently move the focus point of the laser in a third direction, orthogonal to the first two directions.